Re: [PATCH] x86_64: resize NR_IRQS for large machines

[Linus Torvalds <torvalds@linux-foundation.org>]

On Wed, 26 Mar 2008, Pavel Machek wrote:
> 
> This is very ugly. Why not include it unconditionally -- with guard in
> apicdef.h?

I do agree that it's ugly, but I think the ugliness is more serious than 
that.

What I think we should do is to make NR_IRQS no longer be a compile-time 
constant, but instead just do something like

	unsigned int NR_IRQS __read_mostly;

and then just set it early in the boot sequence depending on the real CPU 
numbers etc.

I realize that this will require some changes to a few arrays that are 
statically allocated and depend on NR_IRQ's (notably "irq_desc"), but 
don't you guys think that this would be a cleaner thing?

[ I suspect that irq_desc[] itself could quite reasonably be a rather much 
  smaller __read_mostly hash-table of dynamically allocated entries - the 
  thing would be only modified at boot, so it should cache beautifully 
  even across hundreds of CPU's ]

Whatever. I'm not opposed to this whole static thing, but I do wonder if 
it's worth doing that way. There *may* be performance reasons for doing it 
the way we're doing it, but quite frankly, I think the #define is mostly 
purely historical, from when it was just a fixed number (originally 16!) 
and it made sense to think of it as a small static array.

			Linus